1. Field of the Invention
The present invention relates to a detecting apparatus used for an image forming apparatus such as a copier, a printer, or a facsimile, to which an electrophotographic process or an electrostatic recording process is applied. In particular, the present invention relates to an image information detection apparatus, which is suited for detecting image information on a pattern image for position detection formed on an image bearing member.
2. Description of the Related Art
In a conventional image forming apparatus for obtaining a multiple color image, images having different colors are formed in multiple image forming sections, paper is conveyed by a conveyer unit such as a conveyer belt, and the images are transferred onto the paper while being overlapped, to thereby perform multiple color image formation. In a case of obtaining a full color image by performing multiple color developing, even a slight overlap positional deviation of the paper deteriorates image quality. For example, in a case of a resolution of 400 dpi, one pixel is set to 63.5 μm, and even an overlap positional deviation of a fraction of this value appears as a change of a color misregistration or a color drift and significantly deteriorates an image.
In the initial stage of development of a color image forming apparatus, the multiple color developing is performed by optical scanning with a single image forming section, that is, an image forming section having the same optical characteristic, thereby alleviating the overlap positional deviation of images. However, this method has a problem of requiring much time to output a multiplexed image or a full color image. In order to solve this problem, there is known a color image forming apparatus employing a method of forming images by using optical scanners for respective colors so as to separately obtain images of the respective colors and overlaying the images of the respective colors one on another on the paper conveyed by a conveyer section (see Japanese Patent Application Laid-Open No. 2000-089540 and Japanese Patent Application Laid-Open No. 2002-055497). However, in this method, a color misregistration has been a big problem, which is caused when overlaying the images of the respective colors one on another on the paper.
Conventionally, in many color image forming apparatuses, an image information detection apparatus for detecting the color misregistration at this time is mounted. In the image information detection apparatus, a light beam emitted from a light source unit is condensed at an illumination lens, and a pattern image for position detection depicted on a recording material (image bearing member) being a transferring belt is illuminated with the condensed light beam. Then, image information such as position information or density information on a recording member and a pattern image depicted thereon is detected by a light receiving unit via an imaging lens. The image information on the pattern image is detected according to detection signals detected by the light receiving unit and the driving of an image forming section (image forming unit) is controlled to output images of respective colors.
In recent years, there is an increasingly vocal demand for overall downsizing/simplification of a color image forming apparatus. This also increases a demand for overall downsizing/simplification of an image information detection apparatus. The color image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-089540 achieves the downsizing/simplification by using an illumination optical element (illumination lens) and an imaging optical element (imaging lens) as the same part. Further, the detecting apparatus disclosed in Japanese Patent Application Laid-Open No. 2002-055497 achieves the simplification by unitarily forming the illumination optical element and the imaging optical element as one member.
As one method for achieving the downsizing and simplification of the image information detection apparatus, there is a method of reducing an angle (incident angle of an illumination optical system) between a principal ray of the illumination optical system for illuminating the pattern image and a surface normal to the recording member. This downsizes an illumination optical element and facilitates molding of an element. However, this method is subject to a large number of constraints in terms of layout because a substrate of a light source unit and a substrate of the light receiving unit are in so close vicinity to each other as to cause interference therebetween.
On the other hand, it is possible to avoid the interference between the substrates by increasing a distance between the light source unit and the recording member, but an optical path length becomes longer, which causes the overall apparatus to become larger. Further, if the substrates are brought into close vicinity to each other with less space therebetween, it becomes difficult to appropriately adjust positions of respective members, which causes the occurrence of a large number of color misregistrations.